1. Field of the Invention
The present invention relates to a large-size super twisted nematic (STN) liquid crystal display apparatus used for displays adopted into office automation (OA) apparatuses such as word processors and personal computers, and for liquid crystal monitors proposed as substitutes for cathode-ray tubes (CRTs).
2. Description of the Related Art
A conventional STN liquid crystal display apparatus includes a pair of polarizing plates, a liquid crystal display device sandwiched between the pair of polarizing plates, at least one phase difference plate disposed between the polarizing plate and the liquid crystal display device, and a light diffusing plate disposed, for example, on the outer side of the polarizing plate, i.e. on the viewer's side. The liquid crystal display device includes a pair of glass substrates, a liquid crystal layer sandwiched between the glass substrates, a transparent electrode formed on the liquid crystal layer side surface of each glass substrate, and an orientation film formed on the liquid crystal layer side surface of each transparent electrode. On the side of the liquid crystal display apparatus opposite to the viewer's side is disposed a backlighting system in which a prism sheet, a sheet diffusing plate, a light director and a light source is disposed in this order from the side of the liquid crystal display apparatus.
A challenge for STN liquid crystal display apparatuses is to widen the viewing angle. The backlighting system disposed under the liquid crystal display apparatus typically includes one or two prism sheets in order to improve the front luminance, i.e. the luminance in the direction vertical to the display screen. The addition of one prism sheet increases the front luminance by approximately 70%. However, since the diffusion of the light in the direction orthogonal to the length of the grooves formed in the prism sheet depends on the vertical angle of the grooves, the viewing angle decreases according to the structure of the grooves. To solve the problem that the viewing angle decreases, a prior art widens the viewing angle by using a light diffusing plate and a three-dimensional phase difference plate whose refractive indices in the three-dimensional directions are controlled.
The arts disclosed in Japanese Unexamined Patent Publication JP-A 6-194648(1994) and Japanese Unexamined Patent Publication of Translation JP-A 7-509327(1995) use the light diffusing plate. In Japanese Unexamined Patent Publication JP-A 6-194648 (1994), a prism sheet whose upper surface is a prism surface and whose bottom surface is a smooth surface is disposed on the back surface side of the liquid crystal display device, and the light diffusing plate is disposed on the side of the liquid crystal display device opposite to the backlighting system. The light aligned by the prism sheet to be directed in a predetermined direction is diffused in all directions by the light diffusing plate, so that image information is obtained where the contrast and tone are similar to some extent over a wide range of viewing angle.
In Japanese Unexamined Patent Publication of Translation JP-A 7-509327 (1995), means for reflecting or means for diffusing is provided on the back surface side of the liquid crystal display device and a waveguide having a taper is disposed on the obverse surface of the liquid crystal display device.
The art disclosed in Japanese Unexamined Patent Publication JP-A 5-157911 (1993) uses the three-dimensional phase difference plate. In the art, a polarizing plate is disposed at least on one side of the liquid crystal display apparatus through at least one three-dimensional phase difference plate comprising a birefringent film with refractive indices set so that nx&gt;nz&gt;ny.
The prior art disclosed in Japanese Unexamined Patent Publication JP-A 8-122755 (1996) relates to a liquid crystal display apparatus of low viewing angle dependency, in which a liquid crystal cell is interposed between a pair of polarizing plates, light introducing means for introducing light into the liquid crystal cell is disposed on the external side of one of the pair of polarizing plates, and light diffusing means for diffusing light outgoing from the liquid crystal cell is disposed on the external side of the other of the pair of polarizing plate. The light diffusing means includes a first light diffusing means made of a polymer film, for selectively diffusing light of a predetermined direction which outgoes from the liquid crystal cell, and a second light diffusing means made of a polymer film, for selectively diffusing light of a predetermined direction out of the light diffused by the first light diffusing means. For example, the first light diffusing means diffuses light of a vertical direction selectively, and the second light diffusing means diffuses light of a lateral direction selectively. A vision control film (under a trade name of "Lumisty") manufactured by Sumitomo Chemical Co., Ltd. is used as a polymer film for such means.
The liquid crystal display apparatuses of the prior art are improved in viewing angle by using the light diffusing plate or the three dimensional phase difference plate to obtain image information whose contrast and color tone are substantially uniform over a wide range. In the arts using the conventional light diffusing plate and three-dimensional phase difference plate, however, since the light which enters the front faces of the plates perpendicularly is also diffused in all directions, the contrast in the direction perpendicular to the front face of the liquid crystal display apparatus deteriorates. In the case where the backlighting system includes a prism sheet to ensure a sufficient front luminance, the viewing angle is considerably narrowed.